FIELD OF THE INVENTION
The invention relates to a method for the diagnosis of a continuous-action lambda probe disposed upstream of a catalytic converter of an internal combustion engine.
For controlling the mixture in an internal combustion engine, it is known to provide in the exhaust flow upstream of the catalytic converter, serving for the conversion of harmful exhaust gas constituents, an oxygen sensor or probe, the output signal of which changes in dependence on an oxygen concentration in the exhaust gas.
In addition to so-called sudden-change probes, also referred to as binary probes, the output signal of which changes suddenly both when there is a transition from a rich mixture to a lean mixture and when there is a transition from a lean mixture to a rich mixture (sudden voltage change with the air-fuel coefficient .lambda.=1), oxygen probes with a continuous characteristic curve are also used. These have a continuous, for example linear, dependence of the output signal on the air-fuel coefficient .lambda. and, in addition, a low rise time. (SAE Paper 940149 "Automatic Control of Cylinder Air-Fuel Mixture Using a Proportional Exhaust Gas Sensor").
Such an oxygen probe with a continuous output characteristic, referred to in the following simply as a continuous-action lambda probe, is constructed for example on the basis of strontium titanate (SrTiO.sub.3) by thin-film technology (VDI Berichte 939, Dusseldorf 1992, "Vergleich der Ansprechgeschwindigkeit von KFZ Abgassensoren zur schnellen Lambdamessung auf der Grundlage von ausgewahlten Metalloxiddunnfilmen" [Comparison Of The Rate Of Response Of Motor Vehicle Exhaust Sensors For Rapid Lambda Measurement On The Basis Of Selected Metal-Oxide Thin Films]).
The use of a continuous-action lambda probe results in the changeover from two-point lambda control to continuous lambda control. In order not to exceed the statutory exhaust-emission limit values, the failure of exhaust-relevant components must be detected and indicated (onboard diagnostics).
It is therefore necessary to check the functioning capability of the lambda probes as well. It is known from Published European Patent Application EP 0 616 119 A1 in the case of a lambda probe disposed upstream of the catalytic converter (pre-cat probe) to measure the switching times within which the output signal of the lambda probe switches over during its sudden-changing function from the high voltage value, which indicates a rich mixture (rich voltage), to a low voltage value, which indicates a lean mixture (lean voltage). The magnitude of these switching times is a measure of the functioning capability of the lambda probe disposed upstream of the catalytic converter.
A further method for checking the dynamic functioning capability of pre-cat lambda probes is described in Published European Patent EP 0 652 358 A2. There, the holding times within which the lambda probe signal indicates a rich or lean mixture are measured. The pre-cat lambda probe is then classified as operating correctly if both the rich and the lean holding times are less than prescribed limit values assigned to the individual holding times.